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The volumes of a callosal subregions terminating in the language-related posterior brain regions predict a stronger degree of language lateralization

Jan 27, 2023 | Neuroscience

The research team from HSE University, Lomonosov State University, and Institute of Linguistics, Russian Academy of Sciences, Moscow, Russia, conducted this first tractography study that examined the association between a degree of language lateralization and the volumes and microstructural properties of the corpus callosum (CC) subregions. The results demonstrated that the volumes of callosal subregions that terminate in the language-related posterior brain regions predict a stronger degree of language lateralization.

About the transcallosal cortico-cortical transmission

From the first observations made in the late 19th century, generations of scientists became interested in the origin and consequences of brain asymmetry. Pioneering studies have pointed to the left hemisphere dominance in language, and the right hemisphere dominance in the affective, prosodic, and intonational aspects of spoken language and emotional word processing. Later studies have also documented a dominance of the left hemisphere for language in most humans.

However, about 10–15% of individuals show atypical dominance for language in the right hemisphere or no clear hemispheric dominance. This is more frequently represented in the left-handed and ambidexters than in the right-handed people.

The corpus callosum (CC), as the largest inter-hemispheric commissure, connects the cortical zone of the left hemisphere and the right hemisphere. The anterior regions of the CC contain axons from the frontal, premotor, and motor cortices, and the posterior regions of the CC contain fibers from the somesthetic, parietal, occipital, and temporal lobes.

Transcallosal cortico-cortical transmission is mainly excitatory, but the main and longer-lasting effect in the contralateral hemisphere is inhibitory, probably because most excitatory callosal fibers terminate on pyramidal neurons, which then activate inhibitory interneurons. These activated inhibitory cells may then induce a widespread inhibition in homotopic regions of contralateral neurons.

The role of the CC has two major aspects. The first aspect suggests that CC is associated with functional cortical lateralization at the level of inter-hemispheric inhibition. The second implies that CC contributes to brain symmetry because the impairment of CC leads to intra-hemispheric isolation. Accordingly, two models explain how the CC can contribute to language lateralization. Since most CC fibers rely on excitatory glutamate neurotransmitters, the excitatory model suggests the functional activation of both hemispheres through the CC. The inhibitory model suggests that the dominant hemisphere suppresses the subdominant hemisphere during language tasks through the inhibitory interneurons.

The authors emphasized that over the past decades, efforts have been made to find anatomical correlates for language lateralization in gray and white matter structures. Among gray matter structures, insular asymmetry has been suggested to predict language lateralization. However, it has been shown that asymmetry in areas related to the language, the planum temporale and Broca’s area, does not correlate with the lateralization of the language.

Early tractography studies of language lateralization investigated only the microstructural properties obtained with DTI. However, neither of these studies examined volumes of the callosal subregions.

About the study

The study investigated the relationship between the structural properties of each callosal subregion and the degree of language lateralization in the corresponding cortical area using diffusion tensor imaging (DTI) and constrained spherical deconvolution (CSD) techniques. The CSD is an advanced tractography technique for modeling the CC crossing fibers. This investigation examined for the first time the association between the volumes of the callosal subregions and a degree of language lateralization.

The study included 50 neurologically healthy individuals, 20 were right-handed, another 20 were left-handed, and 10 participants were ambidextrous. They performed a block-designed language task with alternating sentence completion, which activates both anterior and posterior language-related areas.

The brain activity was recorded during the language task using functional magnetic resonance imaging (fMRI). Two tractography techniques, DTI and CSD were utilized to measure the volumes and microstructural properties of the callosal subregions in each participant.

Results

The CSD findings revealed a significantly larger volume for each callosal subregion than DTI. This is consistent with some earlier work which showed that CSD can provide a more complete reconstruction of the crossing callosal fibers. In addition, study results demonstrated that the microstructural properties of callosal fibers did not influence the degree of language lateralization, regardless of the tractography method.

The variability of the microstructural properties and the potential functional specialization of the callosal subregions led the researchers to analyze the degree of language lateralization in each cortical area that corresponds to the specific callosal subregions. The CSD-based analysis found a significant impact only in the area that included language-related posterior brain regions. However, the analysis of DTI-based metrics did not show any significant association with language lateralization.

Specifically, the volumes of callosal subregions terminating in the posterior parietal, temporal, and occipital lobes predicted a stronger degree of language lateralization. According to the authors, these findings support the specific inhibitory model implemented through the callosal fibers projected into the language-related core posterior areas, with no relevant contribution from other callosal subregions. In addition, the relationship between the volume of CC fibers projecting into the parietal, temporal, and occipital lobes and the degree of language lateralization can be explained by the role of the posterior callosal subregion in the comprehension of language.

In conclusion, this first tractography study investigated the relation between the volumes and microstructural properties of callosal subregions and the degree of language lateralization, using both DTI and CSD. In line with the inhibitory model, greater volumes in the CSD, but not in the DTI, predicted a stronger degree of language lateralization in the language-related posterior brain regions, the posterior parietal, temporal, and occipital lobes. This investigation showed that the influence of callosal fibers on the degree of language lateralization is not equipotential, but rather anatomically specific.

This article was published in Plos ONE.

Journal Reference

Karpychev V et al. Greater volumes of a callosal sub-region terminating in posterior language-related areas predict a stronger degree of language lateralization: A tractography study. PLoS ONE 2022; 17(12): e0276721. (Open Access).  https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0276721

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